FR2616810A1 - ELECTROCHEMICAL PROCESS FOR RECOVERING METAL RHODIUM FROM AQUEOUS SOLUTIONS OF USED CATALYSTS - Google Patents

Abstract

Metallic rhodium is recovered from aqueous solutions of spent catalysts by acidification and oxidation followed by electrolysis.

Description

26 168 10

  The present invention relates to an electrochemical process for recovering rhodium in metallic form from solutions

of used catalysts.

  Rhodium is a metal which is used as a catalyst, generally in the form of salts or complexes, to produce

  numerous syntheses in organic chemistry.

  Rhodium is a relatively rare metal with a high cost. It is particularly important to be able to recover it when the catalyst has lost its efficiency in order to be able to recover it.

to recycle.

  It is known to recover rhodium from complexes soluble in organic medium. Generally, the known methods consist in destroying the catalyst (oxidation, acid or alkaline attack, displacement by a stronger ligand, combustion) to obtain a recoverable form (soluble or insoluble rhodium mineral salts, complexes of sufficient polarity) from which the

  rhodium is recovered (electrochemical reduction, filtration, fuel-

  tion, liquid / liquid extraction, fixing on support).

  However, in the case of organic rhodium complexes, the electrochemical reduction does not lead to metallic rhodium but to derivatives in which the rhodium is in the "O" oxidation state (see, for example, E. Markrlik et al , J. Organomet. Chem., 1977, 142 (1), 95-103). It is however possible to obtain the deposit of metallic rhodium on an electrode by using mineral salts or mineral complexes of rhodium in the presence of judiciously chosen electrolytes (see, for example, Chem. Abstr. 1984, 100,

147437 q).

  It is known to use hydro-catalytic systems.

  lubles consisting of a rhodium derivative, mineral or organometallic

  and a water-soluble phosphine (sodium salt of triphenyl-

  phosphinetrisulfonée or TPPTS Na) for. carry out chemical reactions. For example, using such a catalyst, the selective addition of a compound having an active methylene group, such as a -cetoester, can be carried out on a conjugated diene

  terminal (European patent EP 44771), the addition of a secondary amine

  cyclic daire on a terminal conjugated diene (European patent EP 185,559) or else the selective alkylation of phenols (European patent application EP 161,132) or else the hydroformylation of olefins (FR 2,349,562). Generally, at the end of the reaction, the aqueous phase,

  whose pH is close to neutral, is essential

  Rhodium in the form of organometallic complexes, sulfonated phosphines and their degradation products (phosphine oxides), water, alcohol (methanol) used as co-solvent and organic products (reactants and reaction products ). Although the catalyst can be recycled, it must be regenerated after

  a specified number of operations.

  It has now been found, and this is the subject of the present invention, that rhodium can be recovered in metallic form by electrochemical reduction of an aqueous solution of a water-soluble catalytic system, essentially containing a derived from rhodium, mineral or organometallic, a phosphine soluble in water and its degradation products, having previously undergone

  appropriate oxidation and acidification treatment.

  The purpose of the oxidizing treatment is to transform the residual trivalent phosphorus into pentavalent phosphorus and to oxidize the monovalent rhodium to trivalent rhodium, the acidification being

  necessary for the rhodium to be in an electro-reduced form

tible.

  It is particularly advantageous to carry out the pre-

  treatment with an aqueous solution of previously used catalyst

  concentrated in order to have a solution having a concentration

  higher rhodium fractionation from which organic solvents and reaction products and residual reagents have been removed. Generally, a concentrated solution is used, the volume of which represents approximately one third of the initial volume so as to have a rhodium concentration of between 1 and 5 g / liter. The concentration is carried out by distillation optionally under

reduced pressure.

  Oxidation of the spent catalyst solution, if any-

  Lely concentrated, is carried out using an agent having a strong oxidizing power, preferably chosen from hypochlorites or chlorates of alkali metals, such as bleach, sodium or potassium chlorate, oxygenated water. operating at a temperature

  between 20 C and the reflux temperature of the reaction mixture

  tional. Preferably, the oxidation is carried out at the temperature of

  reflux and is complete after a period of from 1 to 4 hours.

  The acidification is carried out using a strong acid so that the pH of the final solution is less than 3 and,

  preferably between 0 and 1. It is particularly advantageous

  gous to use a strong mineral acid chosen from chlorine acids

  hydric, sulfuric, nitric and phosphoric. Preferably, the acidification is carried out at the reflux temperature of the mixture

  reaction, the heating being continued for 1 to 4 hours.

  For the implementation of the pretreatment operation of the aqueous solution of the used catalytic system, it is immaterial

  that the acidification follows or precedes the oxidation.

  The solution which is obtained after the oxidative treatments

  your acidification is directly electrolyzable.

  The electrolysis can be carried out continuously or discontinuously. The electrolyser consists essentially of a cathode

  and an anode and possibly a diaphragm separating the compar-

  cathodic and anodic. The electrolyser may further include a reference electrode such as a saturated calomel electrode. Generally, the cathode is made of a material

  conductor of electricity whose melting point must be sufficient

  low enough to allow rhodium to be recovered and which must be resistant to the aggressiveness of the environment. It is particularly advantageous to use a cathode made of stainless steel, mercury, copper or lead. Preferably, a copper cathode is used

2 6 1 6810

or lead.

  Generally, the anode consists of an electrically conductive material which cannot be attacked under the conditions of electrolysis. It is particularly advantageous to use a graphite anode. When a separating diaphragm is used, it is advantageously constituted by a porous material such as for example a sintered glass plate or an ion exchange membrane, preferably a cation exchange membrane. When using a separator diaphragm, the anode compartment contains an electrolyte which, preferably, is the acid used to effect acidification during

of the pre-treatment operation.

  When the electrolyser does not have a reference electrode, the current density at the start of the electrolysis is determined in such a way that the reduction of rhodium is maximum while limiting the reduction of the protons present in the strongly acid medium. Generally, the current density is between 0.5 and 2 A / dm2. Due to the simultaneous reduction of part of the protons present in the medium, the electrical efficiency is not quantitative. Generally, electrolysis is stopped after

  passage of 50 Faradays per gram atom of rhodium.

  When the electrolyser includes a reference electrode (saturated calomel electrode), the potential imposed on the cathode is chosen in such a way that the reduction of rhodium is maximum while limiting the reduction of the protons present in the strongly acid medium. Generally, the potential of the cathode is understood

  between -0.25 and -0.55 volts relative to the reference electrode.

  For the implementation of the method according to the invention, an electrolyser is generally used in which the anode, the cathode

  and the separating diaphragm are in vertical parallel planes.

  However, when a mercury cathode is used, the anode, the cathode and the separator diaphragm are in horizontal parallel planes.

2616B10

  It is possible to use an electrolyser comprising several anodes and cathodes arranged alternately and connected to

  power supply by parallel circuits.

  Furthermore, it is possible to combine several elementary electrolysers in series. It is particularly advantageous to agitate the solution to be electrolyzed either by means of a mechanical or magnetic stirrer

  either by circulation provided by a pump.

  It is also possible to use electrolysers

  which are attached additional devices such as heat exchangers, expansion vessels or devices for measuring the

temperature or pH.

  When the electrolysis is complete, the cathode (s) are removed from the solution then rinsed with water and optionally with an organic solvent such as methanol or acetone and finally dried. Depending on the adhesion capacities of the deposit on the cathode, it is recovered by mechanical scraping or detached under

  the action of ultrasound in an aqueous medium.

  Generally, the rhodium initially present in the solution is recovered with a yield greater than 80% under a

  solid form grading more than 50% (w / w) of rhodium.

  The metallic rhodium can be recovered either by melting the cathode or by melting the deposit obtained after scraping the

  cathode, the cathode can in the latter case be reused.

  The rhodium in metallic form thus obtained can be

  refined, according to known techniques.

  In this way, it is not necessary to use a specific reducing agent usually used to precipitate the

  rhodium such as borohydride, zinc or iron.

  The rhodium obtained according to the process of the present invention can optionally be transformed into a salt (chloride, bromide, sulfate, nitrate) which can be used to prepare a complex useful as

catalyst in organic synthesis.

Z616810

  The following examples, given without limitation,

illustrate the present invention.

EXAMPLE 1

  1 - Pretreatment In a 100 cm 3 Erlenmeyer flask, 10 cm 3 of a used catalyst solution are introduced containing: rhodiumtriphenylphosphinetrisulfonated complex (2.3 g / liter of rhodium) triphenylphosphinetrisulfonated, sodium salt and oxidation products (40 , 9% as dry extract) organic products (1.4%) - 25 cm3 of an aqueous solution of sodium hypochlorite at IS% (d = 1.22) The mixture is heated in a bain-marie at 80-82 C for 1 hour

20 minutes while shaking.

  1.5 cm3 of 37% hydrochloric acid are then added.

(d = 1.19).

  Heating and stirring are maintained for 1 hour

minutes.

  The precipitate which forms (0.152 g) is separated from the solution by filtration on sintered glass. The precipitate contains 0.064% (w / w) of rhodium which is recovered by combustion according to the

known techniques.

  2 - Electrolysis 25 cm3 of the pre-treated solution are introduced into the cathode compartment of an electrolysis cell, both of which

  compartments are separated by a cationic membrane.

  In the cathode compartment, we introduce a cathode

Z616810

  copper in the form of a disc 2.4 cm in diameter. The compartment

  anodic ment contains 25 cm3 of hydrochloric acid N; we intro

  produces a platinum grid of the same size. The potential of the cathode is fixed at -0.3 volts with respect to a reference electrode with saturated calomel. The solution is stirred. After the passage of 359 coulombs, the electrolysis is stopped. The copper cathode is covered with a gray deposit which is recovered by scraping, rinsed with water and

acetone and then dried.

  3 - Results The initial solution contained 16.25 mg of rhodium and the final solution after electrolysis contains 2 mg. The rate of

recovery is 88%.

  The recovered deposit contains 46% rhodium and 39% copper.

EXAMPLE 2

  i - Pretreatment In a 1000 cm3 Erlenmeyer flask, the following are introduced: - 100 cm3 of used catalyst solution identical to that used in Example 1, and

  - 300 cm3 of 15% sodium hypochlorite.

  Stir and maintain in a water bath at 90-94 C for 1 hour 35 minutes. 15 cm3 of 37% hydrochloric acid (d = 1.19) are added. Stirring and heating are maintained for

  minutes. 0.508 g of red precipitate is collected by filtration.

  pasty mass containing 0.3% rhodium which is recovered by combustion. The volume of the filtrate is brought to 500 cm3 by addition of water. 2 Electrolysis 250 cm3 of pre-treated solution are introduced into an electrolysis cell without a separating diaphragm. A copper cathode and 2 graphite anodes with an area of 25 cm 2 are plunged on either side and equal distance from the cathode. The voltage across the circuit is fixed at 2.66-2.9 volts. The solution is constantly agitated. After passing 2530 coulombs, the electrolysis is stopped. The copper cathode is covered with a strong gray deposit

rhodium content.

  3 - Results The initial solution contained 184 mg of rhodium and the final solution, after electrolysis, contains 25 mg. The rate of

recovery is 86%.

EXAMPLE 3

  1 - Pretreatment In a 100 cm3 Erlenmeyer flask, the following are introduced: - 10 cm3 of used catalyst solution identical to that used in Example 1, and - 14 cm3 of deionized water - 2 cm3 of hydrogen peroxide at 110 volumes One agitates and maintains in a water bath at 87-95 C for 1 hour 30 minutes. 1.2 cm3 of 37% hydrochloric acid (d = 1.19) are added. Stirring and heating are continued for minutes. 0.015 g of pasty reddish precipitate containing 0.35% of rhodium is collected by filtration, which is recovered by combustion.

  The volume of the filtrate is brought to 30 cm3 by addition of water.

  2 - Electrolysis 28 cm3 of pre-treated solution are introduced into an electrolysis cell without a separating diaphragm. A copper cathode and a 4.5 cm2 graphite anode are placed face to face and vertically. The current intensity is initially set to mA. The solution is constantly agitated. After passing 918 coulombs, the electrolysis is stopped. The copper cathode is

  covered with a gray deposit containing rhodium.

  3 - Results The initial solution contained 34.2 mg of rhodium and the final solution, after electrolysis, contains 2.1 mg. The rate of

recovery is 94%.

Claims (8)

  1 - Process for recovering rhodium in metallic form   from an aqueous solution of used catalyst containing essen-   tially a rhodium complex, a sulfonated phosphine and its oxidized derivatives, residual organic products, solvents and mineral salts characterized in that: a) the aqueous solution is treated with an oxidizing agent and with a strong acid,   b) subjecting the solution thus obtained to an electrolytic reduction tick then   c) the deposited metallic rhodium is separated from the cathode.   2 - Method according to claim 1 characterized in that   the aqueous solution of the catalytic system is concentrated beforehand-   oxidation and acidification treatment.   3 - Method according to one of claims 1 or 2 characters-   in that the oxidation followed or preceded by the acidification is carried out by means of an oxidizing agent chosen from hypochlorites of   alkali metals, alkali metal chlorates and hydrogen peroxide.   4 - Method according to one of claims 1 or 2 characters-   rised in that the acidification preceded or followed by the oxidation is carried out by means of a strong mineral acid chosen from hydrochloric, sulfuric, nitric or phosphoric acids so as to bring about pH below 3.   - Method according to one of claims 1 or 2 characters-   that the electrolytic reduction is carried out in an electrolyser comprising a cathode, an anode and possibly a   reference electrode and / or a diaphragm separating the compartments   anode and cathode elements, the cathode being made of an electrically conductive material, of low melting point and resistant to corrosion of the medium chosen from stainless steel, mercury, copper and lead, anode consisting of   material that conducts electricity and cannot be attacked under the conditions   electrolysis such as graphite, the reference electrode being a saturated calomel electrode and the separating diaphragm being a porous material chosen from sintered glass and ion exchange membranes.   6 - Method according to claim 5 characterized in that, when operating in the absence of reference electrode, the density of   current at the start of electrolysis is between 0.5 and 2 A / dm2.   7 - Method according to claim 5 characterized in that, when operating in the presence of a reference electrode, the potential of the cathode is between -0.25 and -0.55 volts by   relative to the reference electrode.   8 - Method according to one of claims 5, 6 or 7   characterized in that, when operating in the presence of a separating diaphragm, the anode compartment contains a selected electrolyte   among the acids used during the acidification operation.   9 - Process according to claim 1 characterized in that the metallic rhodium is obtained on the cathode in the form of a deposit   which is separated by mechanical scraping or by the action of ultrasound.